Generally, construction machines like hydraulic excavators are provided with a variable displacement hydraulic pump which constitutes a pressure oil source along with a tank. A rotational shaft of a variable displacement hydraulic pump of this sort is driven from a prime mover like a Diesel engine to supply pressure oil to and from various hydraulic actuators such as working hydraulic cylinders, a vehicle drive hydraulic motor or a revolving hydraulic motor.
There has conventionally been known a variable displacement hydraulic pump of this sort (hereinafter referred to as a first prior art) which is provided with a tilting controller, including tilting actuators to which a tilting control pressure is supplied to drive tilting motions of a volume varying portion of the hydraulic pump, a regulator in the form of a servo valve having a spool within a control sleeve for controlling the tilting control pressure to be supplied to and from the tilting actuators according to a command signal from outside, and a feedback mechanism which is adapted to follow tilting motions of the volume varying portion for feedback control of the control sleeve of the regulator (e.g., Japanese Patent Laid-Open No. 2003-74461).
In the case of the tilting controller just mentioned, as the spool of the regulator is put in a sliding displacement in response to a command signal from outside, the tilting control pressure is switched to let the tilting actuators drive the volume varying portion. The feedback mechanism is arranged, for example, to turn a feedback link, following tilting motions of the volume varying portion. Further, by transmission of a rotational displacement of the feedback link, the control sleeve of the regulator is put in a sliding displacement in the same direction as the spool for the sake of feedback control.
On the other hand, as a second prior art, there has also been known a tilting controller for a variable displacement hydraulic pump, which is arranged for application exclusively to a closed hydraulic circuit (e.g., Japanese Patent Laid-Open No. H5-39863).
In the case of the tilting controller employed in the variable displacement hydraulic pump by the above-mentioned first prior art, a volume varying portion of the hydraulic pump is driven to tilt only in one direction (e.g., in a normal or forward direction), for example, in reference to a zero angle neutral position, without taking into account tilting motions in a reverse direction from the neutral position.
Therefore, in order to connect the variable displacement hydraulic pump to a hydraulic actuator like a hydraulic motor through a closed hydraulic circuit, drastic restructuring becomes necessary for driving the volume varying portion to tilt in both forward and reverse directions from the neutral position.
Besides, when applied to a closed hydraulic circuit, the control sleeve of the regulator has to be fed back (put in sliding displacements) in both forward and reverse directions as the volume varying portion is tilted in forward and reverse directions, and this makes smooth feedback control of the regulator difficult.
On the other hand, in the case of the tilting controller of the variable displacement hydraulic pump by the second prior art which is intended specifically for use with a closed hydraulic circuit, a tilting actuator which drives a volume varying portion of the hydraulic pump is assembled integrally with a volume control valve.
More particularly, in this case of the tilting controller, main spools and pressure chambers are provided on the opposite sides of a regulator piston which functions as a tilting actuator, necessitating an increased number of component parts due to complicate construction of the controller as a whole.
Besides, in this case, the tilting controller which is constructed exclusively for use with a closed hydraulic circuit is too limited in versatility to exclude applications to an open hydraulic circuit, that is to say, can find only limited applications as a hydraulic pump.